Xenotransplantation has been proposed as a solution to the shortage of human organs available for transplantation. The recent availability of inbred miniature swine with homozygous disruption of the a- galactosyltransferase genes (GalT-KO) has enhanced the likelihood that immunologic hurdles to pig-to- primate Xenotransplantation may be overcome. Concerns regarding the clinical application of Xenotransplantation have focused on the risk of infection due to novel organisms from non-human species in xenograft recipients. While no such xenogeneic infections have been identified in humans exposed to pig tissues, previous studies have identified a series of viral pathogens of swine activated within porcine xenografts. These include the family of porcine endogenous retroviruses (PERV) first cloned in this laboratory, as well as porcine cytomegalovirus (PCMV) and porcine lymphotropic herpesvirus (PLHV-1). The goal of this proposal is to examine the risk for infection of primates by porcine pathogens undergoing Xenotransplantation. In particular, we will study the mechanisms underlying production of human-tropic recombinant PERV A-C, xenografts from GalT-KO swine, and the impact of viral infections on graft survival. These studies will utilize long-standing collaborations with on-going studies of tolerance induction across species' barriers in primate and murine models. These studies will extend observations on xenogeneic infections in three areas. Specific Aim 1: To characterize mechanisms for the generation of human-tropic recombinant porcine endogenous retrovirus (PERV A-C) and the effects on virus production of tolerance induction strategies. Specific Aim 2: To determine whether the risk of infection by porcine endogenous retrovirus for primates and human cells, if any, is increased using xenografts from GalT-KO swine. Specific Aim 3: To assess the role of other viral infections on graft survival and PERV production in Xenotransplantation. These data will examine molecular and cellular mechanisms associated with the production of potential human pathogens and may allow the exclusion of these pathogens from miniature swine used in future clinical trials.